In order to ensure compliance with customer specifications and government regulations, most electrical devices undergo a series of tests to ensure that they are operating within established limits. One class of electrical devices is that of commutator type electrical motors or generators. Motors or generators of this type generally employ a plurality of armature coils or windings which are connected in series. The ends of the coils re each connected to a commutator bar, which in turn, are brought in and out of contact with brush type contacts to thereby intermittently connect the coils to an electric power source for a motor or the electrical load for a generator.
The connections between each of the commutator bars and its associated coil is typically in the form of a weld or a solder joint and when testing the armature for winding anomalies and/or a defective weld connection, there can be significant testing problems. One of the inherent difficulties encountered during such testing is the large difference in values between the coil resistance itself and the resistance of the weld connection. For example, the resistance of a typical armature coil may be 0.5 ohms while the resistance of a weld may range from 40 micro ohms (for a ideal connection) to 500 micro ohms (for a non-open but unreliable weld). As a result of such great differences in resistance, the direct measurement of the resistance between two commutator bars cannot be relied upon to detect a defective connection between the coil and the commutator bar. For example, during manufacture the weld may be formed with a small discontinuity or void which, although not severe enough to cause an open circuit (large resistance) and thus be readily detectable, will be likely nonetheless to shorten the operating life of the motor. However, if each of the weld resistances can be accurately determined, then each such resistance can be compared with standard values to determine whether the weld is acceptable or must be rejected.
U.S. Pat. No. 4,651,086 to Axis S.p.A. discloses a method and apparatus for determining electrical resistance between the coils and commutator bars as well as the coil resistances themselves. As shown in FIG. 1 of this patent, a first d.c. current, I.sub.1, is passed through a first set of commutator bars 1 and 2, and a second current source, I.sub.2, is passed through a second set of commutator bars 3 and 2. Current I.sub.2 is a dependent current source which is controlled by a feedback loop so as to null or completely cancel the currents flowing through the other armature coils (R.sub.B6, R.sub.B5, R.sub.B4, and R.sub.B3). As a result of no current flow through R.sub.B6, a voltmeter can measure the voltage drop between commutator bars 1 and 6 which is really the voltage drop across R.sub.S1. As such, the resistance of the weld, R.sub.S1 can be calculated using the known value of I.sub.1. As a practical matter, the method of the '096 patent attempts to completely eliminate the affect of the armature coil resistances so that direct measurements of the voltage drop across the welds, per se, can be obtained.
The method and apparatus of the '086 patent, however, have several disadvantages. In particular, the method requires the use of a cancelling current in the form of a dependent current, I.sub.2, which may be difficult and expensive to control, especially in a production environment where noise is a significant problem. As a result, any current flow through R.sub.B6 will affect the accuracy of the measured weld resistance R.sub.S1. Further, the use of an additional current source can add noise which can induce errors into the measurement of the weld resistances.